**Core Concept**
The Haase rule is a simple method used in obstetrics to estimate the age of a fetus. This rule is based on the assumption that the average length of the fetus increases in a predictable manner during the last five months of gestation.

**Why the Correct Answer is Right**
The Haase rule estimates fetal age by measuring the crown-rump length (CRL) of the fetus, which is the distance between the top of the head and the buttocks. The rule states that for every 4 weeks of gestation, the fetus grows 1 cm in length. Therefore, for every month of gestation, the fetus grows approximately 2.5 cm in length. This linear growth pattern allows for a relatively accurate estimation of fetal age using the Haase rule.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the Haase rule, which specifically deals with the last five months of gestation.
* **Option B:** This option is not a correct application of the Haase rule, which is based on the crown-rump length of the fetus.
* **Option C:** This option is not a correct formula for the Haase rule, which is based on a linear growth pattern of the fetus.

**Clinical Pearl / High-Yield Fact**
The Haase rule is a useful tool for estimating fetal age during the last five months of gestation. However, it is essential to remember that this rule is based on average growth patterns and may not be accurate in cases of fetal growth restriction or other abnormalities.

**Correct Answer: C. The Haase rule estimates fetal age as follows: Age (months) = CRL (cm) / 2.5**